Add a check of the current behaviour importing duplicates

- We don't check if the same config is imported multiple times

cabal-testsuite/PackageTests/ConditionalAndImport/cabal.out

@@ -124,6 +124,9 @@ Could not resolve dependencies:
       (constraint from oops-0.project requires ==1.4.3.0)
 [__1] fail (backjumping, conflict set: hashable, oops)
 After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: hashable (3), oops (2)
+# checking if we detect when the same config is imported via many different paths (we don't)
+# cabal v2-build
+Up to date
 # checking bad conditional
 # cabal v2-build
 Error: [Cabal-7090]

cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs

@@ -225,6 +225,39 @@ main = cabalTest . withRepo "repo" . recordMode RecordMarked $ do
     \        imported by: oops-0.project")
     oopsing
 
+  -- The project is named yops as it is like hops but with y's for forks.
+  -- +-- yops-0.project
+  --  +-- yops/yops-1.config
+  --   +-- yops-2.config
+  --    +-- yops/yops-3.config
+  --     +-- yops-4.config
+  --      +-- yops/yops-5.config
+  --       +-- yops-6.config
+  --        +-- yops/yops-7.config
+  --         +-- yops-8.config
+  --          +-- yops/yops-9.config (no further imports)
+  --  +-- yops/yops-3.config
+  --   +-- yops-4.config
+  --    +-- yops/yops-5.config
+  --     +-- yops-6.config
+  --      +-- yops/yops-7.config
+  --       +-- yops-8.config
+  --        +-- yops/yops-9.config (no further imports)
+  --  +-- yops/yops-5.config
+  --   +-- yops-6.config
+  --    +-- yops/yops-7.config
+  --     +-- yops-8.config
+  --      +-- yops/yops-9.config (no further imports)
+  --  +-- yops/yops-7.config
+  --   +-- yops-8.config
+  --    +-- yops/yops-9.config (no further imports)
+  --  +-- yops/yops-9.config (no further imports)
+  --
+  -- We don't check and don't error or warn on the same config being imported
+  -- via many different paths.
+  log "checking if we detect when the same config is imported via many different paths (we don't)"
+  yopping <- cabal' "v2-build" [ "--project-file=yops-0.project" ]
+
   log "checking bad conditional"
   badIf <- fails $ cabal' "v2-build" [ "--project-file=bad-conditional.project" ]
   assertOutputContains "Cannot set compiler in a conditional clause of a cabal project file" badIf